1. Field of the Invention
This invention relates to an optical head and disk apparatus which use near field wave, and a method for manufacturing optical heads, and more particularly, relates to an optical head which implements high density recording on a recording medium and a small-sized optical head of improved data transfer rate, a disk apparatus, a method for manufacturing optical heads, and an optical element for an optical head.
2. Description of Related Art
In the field of optical disk apparatus, the optical disk has changed historically from the compact disk (CD) to the digital video disk (DVD), which has a large recording capacity and is capable of high density recording. The recent development of high performance computers and high resolution displays has resulted in increasing demand for large capacity recording.
The recording density of an optical disk depends basically on the diameter of an optical spot formed on a recording medium. Recently, the near field wave technology in the field of the microscope has been applied to the optical recording technology as a technology for miniaturizing the beam spot diameter. As the conventional optical disk apparatus which uses the near field wave, for example, the optical disk described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443) and U.S. Pat. No. 5,497,359 has been known.
FIG. 23(a) and FIG. 21(b) show an optical disk apparatus described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443). As shown in FIG. 23(a), the optical disk apparatus 190 is provided with a semiconductor laser 191 that emits a laser beam 191a, a coupling lens 192 that changes the laser beam 191a emitted from the semiconductor laser 191 to a collimated beam 191b, and an optical fiber 193 which is polished in a taper shape having a larger diameter at the incident end 193a and a smaller diameter at the emission end 193b, and provided with a probe 194 that introduces the collimated beam 191b which comes from the coupling lens 192 from the incident end 193a, and a recording medium 195 on which the information is recorded by means of the near field wave 191c that leaks from the emission end 193b of the optical fiber 193.
The recording medium 195 has a recording layer 195a consisting of GeSbTe, which is a phase change recording medium, which recording medium is heated by incident near field wave 191c, and then the heating causes phase change between crystal/amorphous, and difference in reflectance between both phases is utilized for recording.
The optical fiber 193 has the incident end 193a having a diameter of 10 xcexcm and the emission end 193b having a diameter of 50 nm, and is coated with a metal film 194b consisting of a metal such as aluminum with interposition of a clad 194a to prevent the beam from leaking to somewhere other than the emission end 193b. The diameter of the near field wave 191c has the approximately same diameter as the diameter of the emission end 193b, therefore the high density recording of several 10 Gbits/inch2 is possible.
For reproduction, as shown in FIG. 23(b), a near field wave 191c having such a low power as it does not cause phase change is irradiated onto the recording layer 195a by use of the same optical head as used for recording, the reflected beam 191d from the recording layer 195a is condensed on a photomultiplier 197 by means of a condenser lens.
FIG. 24 shows an optical head of an optical disk apparatus disclosed in U.S. Pat. No. 5,497,359. The optical head 50 is provided with an condense lens 52 that condenses a collimated beam 51 and an Super SIL (Super Solid Immersion Lens) 54 having the form of bottom-cut sphere placed with the bottom plane 54a perpendicular to the condensed beam 53 from the condense lens 52. The collimated beam 51 is condensed by the condense lens 52 and the condensed beam 53 is incident onto the spherical incident surface 54b, the condensed beam 53 is refracted at the incident surface 54b and condensed on the bottom surface 54a to form a beam spot 55 on the bottom surface 54a. Because the wavelength of the beam becomes short in inversely proportional to the refractive index in the internal of the super SIL 54, the diameter of the beam spot becomes small in proportion to it. A part of the beam condensed on the beam spot 55 is totally reflected toward the incident surface 54b, but the beam leaks partially from the beam spot 55 to the outside of the super SIL 54 as a near field wave 57. A recording medium having the approximately same refractive index as that of the super SIL 54 is located at the close distance from the bottom surface 54a so that the distance is sufficiently smaller than a wavelength of the wave, then the near field wave 57 is coupled with the recording medium 56 and propagates in the recording medium 56. The information is recorded on the recording medium 56 by the propagation beam.
By structuring the Super SIL 54 so that the collimated beam 51 is condensed at the position r/n (r denotes the radius of the Super SIL) distant from the center 54c of the semi-spherical surface 54b, the spherical aberration due to the Super SIL 54 is reduced and the numerical aperture in the Super SIL 54 is increased, and further the diameter of the beam spot 55 is minimized. In detail, the beam spot 55 is minimized according to the equation 1.
Dxc2xd=kxcex/(nxc2x7NAi)=kxcex/(n2xc2x7NAo)xe2x80x83xe2x80x83(1)
where,
Dxc2xd: beam spot diameter where the intensity becomes a half of the maximum intensity.
k: proportional constant (normally around 0.5) which depends on the intensity distribution of an optical beam
xcex: wavelength of an optical beam
n: refractive index of an Super SIL 54
NAi: numerical aperture in an Super SIL 54
NAo: numerical aperture of an incident beam to an Super SIL 54
The collimated beam 51 is condensed as the beam spot 55 without absorption on the optical path and high optical utilization factor is obtained. As the result, a beam source having a relatively low output is sufficient for use and the reflected beam is detected without a photomultiplier.
However, according to the conventional optical disk apparatus 190, though a beam spot having a size of several ten nm is formed on a recording medium, a laser beam which enters an optical fiber 193 is partially absorbed in its inside due to the taper shape of the optical fiber 193, and the optical utilization factor is as low as {fraction (1/1000)} or lower disadvantageously. Because of the low optical utilization factor, a photomultiplier 197 is undesirably required to detect the reflected beam 191d and the photomultiplier leads to a large sized as well as expensive optical head. Further, slow response speed of the photomultiplier 197 and heavy weight optical head result in the slowed-down tracking speed. Due to many problems such as a low transfer rate due to slow rotation of an optical disk, much improvement is required for practical use.
FIG. 25 is a graph for describing the problem of the conventional optical head 50 shown in FIG. 24, which was presented by T. Suzuki in #OC-1 in Asia-Pacific Data Storage Conference (Taiwan, 1997. 7), the relation between the refractive index n of a SIL 54 and NAo is shown. There is a reversal relation between NA of incident beam to the SIL 54, namely the maximum value xcex8max of the incident angle xcex8, and the refractive index n of the SIL 54, and the two values cannot be increased independently. It is understandable as shown in the graph that the possible maximum value NAomax of NAo of the incident beam becomes gradually smaller with increasing of the refractive index of the SIL 54, because the beam having a large incident angle which is caused from increased NAo larger than the maximum NAomax enters directly into the recording medium 56 without passing through the SIL 54 and the size of the beam spot 55 positioned at the recording medium 56 becomes larger instead. For example, if n=2, then NAomax is 0.44, the product nxc2x7NAomax does not exceeds the value of 0.8 to 0.9 for any combination of n and NAomax. This value is the theoretical limit and the actual value is smaller (0.7 to 0.8) than the theoretical value.
B. D. Terris et al. presented their test result on the Super SIL in Appl. Phys. Lett., Vol. 68, (1996), P. 141. According to the test report, a laser beam having a wavelength of 0.83 xcexcm was condensed to form a beam spot having a diameter of 0.317 xcexcm by use of a Super SIL having a refractive index n=1.83 placed between an condense lens and a recording medium, that is, Dxc2xd=xcex/2.3 was obtained. In this case, NA is 0.4, nxc2x7NAmax is about 0.73. The experimental result with use of this system suggests the possibility of high recording density as high as 0.38 Gbits/cm2, which is several times that of the conventional system.
In detail, the conventional optical head 50 with a laser beam having a wavelength of 400 nm gives a beam spot having a diameter of 0.2 xcexcm at the best because there is a reversal relation between the refractive index of the SIL and the maximum Naomax, and the theoretical limit of the product nxc2x7NAomax is 0.8 to 0.9, and the actual limit is 0.7 to 0.8 though the optical utilization factor is high. The diameter of the beam spot is several times larger than that of conventional example in which a probe 194 is used for condensing, and thus the conventional optical head 50 is disadvantageous in that the recording density cannot be enhanced.
FIG. 26 shows a conventional optical head described in the literature xe2x80x9cNIKKEI ELECTRONICS (Jul. 15, 1998) (No. 718)xe2x80x9d. The optical head is, called as SIM (Solid Immersion Mirror) type, provided with a transparent condensing medium 101 having a concave incident surface 101a on which a collimated laser beam 2b is incident, a condense plane 101b provided on the position facing to the incident surface 101a, a planer reflecting surface 101c provided on the periphery of the condense plane 101b, and a non-spherical reflecting surface 101d formed on the periphery of the incident surface 101a, a planer reflecting film 102 formed on the surface of a planer reflecting surface 101c, and a non-spherical reflecting film 103 formed on the surface of the non-spherical reflecting surface 101d. A collimated laser beam 2b comes to the incident surface 101a of the transparent condensing medium 101 of the optical head having the structure as described herein above, the incident collimated laser beam 2b which comes to the incident surface 101a is diffused on the incident surface 101a, the diffused beam 2d is reflected on the planer reflecting film 102, the reflected beam 2e is reflected on the non-spherical reflecting film 103, the reflected beam is condensed on the condense plane 101b, and a beam spot 9 is formed on the condense plane 101b. The near field wave 10 which leaks from the condense plane 101b is served for recording and reading on the recording layer 8a of a recording medium 8. The numeral aperture NA of the planer reflecting surface 101c of the transparent condensing medium 101 is around 0.8, and the refractive index of the transparent condensing medium 101 is 1.83, and NA in the transparent condensing medium 101 is approximately 1.5.
The optical head shown in FIG. 26 gives a beam spot having a diameter of as large as 0.35 to 0.39 xcexcm and is disadvantageous in that the recording density cannot be enhanced because of the minimizing limit of the spot diameter formed on a condense plane of the transparent condensing medium.
Accordingly, it is the object of the present invention to provide an optical head and optical disk apparatus which are capable of high density recording on a recording medium and enhancing the size minimization and data transfer rate, and a method for manufacturing the optical head.
To achieve the above-mentioned object, the present invention provides an optical head, comprising a laser emitting a laser beam; an optical system including a transparent condensing medium which has a condense surface and condensing the laser beam to form a beam spot on the condense surface; and a shade positioned on the transparent condensing medium and having a slit, wherein the slit which is longer than the diameter of the beam spot in length and narrower than the diameter of the beam spot in width is positioned at which the beam spot is formed.
To achieve the above-mentioned object, the present invention provides an optical head, an optical head, comprising: a laser emitting a laser beam; a optical condense element condensing the laser beam; a transparent condensing medium having a condense surface on which the condensed laser beam forms a beam spot; and a shade positioned on the transparent condensing medium and having a slit, wherein the slit which is longer than the diameter of the beam spot in length and narrower than the diameter of the beam spot in width is positioned at which the beam spot is formed.
To achieve the above-mentioned object, the present invention provides a disk apparatus, comprising: a rotator which rotates a disk which holds an information, an optical head recited above; and an optical head actuator coupled with the optical head.
To achieve the above-mentioned object, the present invention provides a method for manufacturing an optical head comprising: step in which a transparent condensing medium having a condense plane on which an incident laser beam forms a beam spot is prepared; a step in which a photo-resist having a width almost equal to or smaller than that of the beam spot is formed on the transparent condensing medium; a step in which a concave is formed by removing the area where the photo-resist does not cover on the transparent condensing medium by etching to the predetermined depth smaller than the wavelength of the laser beam, and a step in which a shade film having an slit with a smaller area than the size of the beam spot is formed by depositing a shading material on the concave.
To achieve the above-mentioned object, the present invention provides a method for manufacturing an optical head recited above, comprising a step for forming the shade with the slit on the transparent condensing medium, wherein the shade forming step includes a etching process performed from the condense surface side of the transparent condensing medium.
To achieve the above-mentioned object, the present invention provides an optical element for an optical head, comprising: a incident surface on which a laser beam is incident; and a projection having a width narrower than a diameter of a beam spot of the laser beam, wherein the beam spot s formed on the projection.